Aqueous solutions containing ammonium compounds and especially "fixed ammonia" and optionally acid gases are a common waste water which requries purification. Such a waste water is produced as a by-product from coal carbonization plants. The need for such purification has become increasingly important due to the emphasis today on clean air and clean water. If the waste water is to be discharged into a river or a stream, it is obviously necessary to reduce the noxious or toxic properties of the waste water. On the other hand, in a coal carbonization plant it is often desirable to reuse the waste water to "quench" the hot coke from the coke ovens. If these waste waters contain noxious or toxic materials, then there may be a serious air pollution problem.
A common way of removing impurities from these waste waters is by means of a two-step distillation process using distillation apparatus with a free ammonia and a fixed ammonia section. See "Industrial Chemistry", E. R. Riegel, Reinhold Publishing Corp., N.Y., 1942, pp. 265-268.
One of the serious problems of this two-step distillation process is the fouling of the fixed ammonia still section by deposits of solids formed in the waste waters upon the lime addition. The free ammonia portion of the distillation apparatus removes most of the free ammonia and/or acid gases present in the waste water. The waste water is then treated with lime to convert the fixed ammonia to free ammonia which is then removed in the fixed ammonia still portion of the distillation apparatus. Solids deposit on the trays in the fixed ammonia still so that periodic cleaning is necessary to allow efficient operation of the still. This is costly and time-consuming and also cuts down on the production possible from a given facility. A number of attempts have been made to overcome this problem. In U.S. Pat. No. 2,839,369 an attempt is made to remedy the problem by designing a special still which can be easily cleaned. A further approach is described in U.S. Pat. No. 3,278,423 wherein the waste waters are first treated in a free ammonia still to remove most of the free ammonia, then phenols and the like are removed by biochemical oxidation, then lime is added to the thus treated waste waters to convert the fixed ammonia to free ammonia, then substantially all of the solids are separated from the lime-containing waste water and then the thus treated waste water is steam distilled to remove the free ammonia thus produced. This process has the disadvantage of requiring substantial lime consumption and results in a large amount of sludge which creates serious handling and disposal problems. Furthermore, the biochemical oxidation step may have serious operating problems since many biochemical oxidation plants will not work if the waste water being treated contains more than about 500 parts per million by weight of total ammonia. Total ammonia includes both free ammonia and fixed ammonia. Thus, in cases where significant total ammonia is present, the waste waters would require considerable dilution for efficient operation of the biochemical oxidation part of the process, thus increasing the size of equipment and therefore the cost of such equipment.
The term "fixed ammonia salts" is used herein in its generally accepted meaning to cover those compounds of ammonia which are not decomposed by heat alone at the temperature employed in the ammonia still, but which in alkaline solution yield free ammonia. Generally, in coke-plant waste waters, the major portion of the fixed ammonia is present as ammonium chloride. Other such compounds are ammonium thiosulfate, ammonium thiocyanate, ammonium ferrocyanide and ammonium sulfate. The term "fixed ammonia" refers to the ammonia portion of the fixed ammonia salts.
The term "free ammonia salts" as used herein is meant to include ammonia itself and also those compounds present in waste waters from which ammonia is liberated by heat in the free ammonia still. Such compounds are ammonium carbonate, ammonium bicarbonate, ammonium sulfide, ammonium bisulfide, ammonium cyanide and ammonium carbamate. The term "free ammonia" refers to ammonia or the ammonia portion of the free ammonia salts.
The term "lime" is used herein to include any of the various chemical and physical forms of quicklime (calcium oxide), hydrated lime (calcium hydroxide), and hydraulic lime. The lime may include a number of impurities such as silica, calcium carbonate, magnesium oxide, iron oxide and aluminum oxide. Some of them may be insoluble when added to the waste water.
The term "acid gases" is used herein to include hydrogen cyanide, hydrogen sulfide and carbon dioxide. These gases may be present in the original waste water as weak ammonium salts that completely dissociate into ammonia and their respective acid gases upon steam distillation.
The waste water described herein are generally industrial waste waters produced by the high-temperature cracking of carbonaceous material. Often such carbonaceous materials are coal or petroleum products. Waste water may also be formed in water-scrubbing of coke-oven gases.